It is known that N-vinylformamide is a compound having high polymerizability and useful as a material of polyvinylamine and other water soluble polymers and as an intermediate of organic chemicals. However, N-vinylformamide having a higher purity has been demanded. Typical known production methods of N-vinylformamide include a method in which an alcohol is eliminated from N-(.alpha.-alkoxyethyl)formamide as described in U.S. Pat. No. 3,914,304, a method in which hydrogen cyanide is eliminated from formylalaninenitrile as described in JP-A-61-134359 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and a method in which ethylidenebisformamide is decomposed with heat as described in U.S. Pat. No. 4,490,557 and U.S. Pat. No. 4,578,515. In all of these methods, N-vinylformamide is obtained by thermal decomposition at a high temperature of 100.degree. C. or more under a reduced pressure. From the thermal decomposition reaction solution, a crude N-vinylformamide generally having a purity of 80% by weight or more can be recovered by distillation removal of low boiling point ingredients and the like. The crude N-vinylformamide generally contains eliminated molecules of the precursor, such as alcohols and formamide. In many cases, it also contains various acidic and basic substances by-produced at the time of the thermal decomposition. These impurities exert adverse influences such as reduction of the yield in the subsequent purification step and decrease in the molecular weight in the polymerization or reaction step, to thereby considerably spoil the quality of a final product.
In order to obtain high purity N-vinylformamide by purifying a crude N-vinylformamide, a method in which a crude N-vinylformamide is continuously distilled using a fractionating tower is considered to be most effective. However, since N-vinylformamide has extremely high reactivity, there is a danger of causing a thermal decomposition reaction, a thermal polymerization reaction and the like, which are undesirable side reactions during the distillation. Particularly, the thermal polymerization reaction frequently causes formation of insoluble and infusible polymer compounds, and this results in the choking of a distillation apparatus. As a result, the distillation recovery yield is extremely reduced and a continuous use of the distillation becomes difficult. Such problems occur even under relatively moderate operation conditions of industrial scale continuous distillation, namely under a reduced pressure of generally from 0.1 to 3 KPa as a distillation tower head pressure, which corresponds to the tower head temperature of about 50 to 100.degree. C. as.
Several methods have been proposed as a method for stably effecting distillation of N-vinylformamide, such as a method in which a crude N-vinylformamide is distilled with previously adjusting its pH value, as described in JP-A-62-195352, JP-A-6-122661, and a method in which distillation of a crude N-vinylformamide is carried out with previously adding a specific stabilizing agent, as described in JP-A-61-289068, JP-A-8-48657, etc. Also, JP-A-62-190153 proposes a method in which a crude N-vinylformamide containing 1 to 70% of formamide is subjected to distillation.
However, most of the conventional methods for stably distilling N-vinylformamide are based on the knowledge obtained in laboratory level, i.e., small scale batch distillation tests, so that prevention of thermal polymerization in a long-term continuous distillation is still insufficient, sometimes causing formation of insoluble and infusible polymers. Further, though examples of continuous distillation are known, as described in U.S. Pat. 4,814,505 (corresponding to JP-A-62-190153) and JP-A-5-155829, they have a problem, for example, in that a crude N-vinylformamide contains a large-amount of formamide or that purity of the N-vinylformamide recovered by continuous distillation is still insufficient.
Particularly, whereas a crude N-vinylformamide generally contains a small amount of formamide formed during synthetic steps, the continuous distillation in the case of the example disclosed in the specification of U.S. Pat. No. 4,814,505 is carried out using a crude N-vinylformamide containing about 35% of formamide prepared by adding a large amount of formamide. Such a method may be effective in preventing thermal polymerization, but it is necessary to employ an additional step for the recovery and purification of formamide used in excess amount. Therefore, this method cannot be advantageous for industrial application.